Current demands for high density and performance associated with ultra large scale integration in semiconductor devices require submicron features, increased transistor and circuit speeds, and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring.
One important process requiring careful inspection is photolithography, wherein masks or “reticles” are used to transfer circuitry patterns to semiconductor wafers. Typically, the reticles are in the form of patterned chrome over a transparent substrate. A series of such reticles are employed to project the patterns onto the wafer in a preset sequence. Each photolithographic reticle includes an intricate set of geometric patterns corresponding to the circuit components to be integrated onto the wafer. The transfer of the reticle pattern onto the photoresist layer is performed conventionally by an optical exposure tool such as a scanner or a stepper, which directs light or other radiation through the reticle to expose the photoresist. The photoresist is thereafter developed to form a photoresist mask, and the underlying poly-silicon insulators or metal layer is selectively etched in accordance with the mask to form features such as lines or gates.
From the above description, it should be appreciated that any defect on the reticle, such as extra or missing chrome, may transfer onto the fabricated wafer in a repeated manner. Thus, any defect on the reticle would drastically reduce the yield of the fabrication line. Therefore, it is of utmost importance to inspect the reticles and detect any defects thereupon. The inspection is generally performed by an optical system, using transmitted, reflected, or both types of illuminations. An example of such a system is the Aera™ series reticle inspection system available from Applied Materials of Santa Clara, Calif.
There are several different known algorithm methods for inspection of reticles. These methods include: “Die to Die” inspection, in which a die is compared to a purportedly identical die on the same reticle; or “Die to Database” inspection, in which data pertaining to a given die is compared to information in a database, which could be the one from which the reticle was generated. Another inspection method involves Die to golden die which is a die chosen as a reference for inspecting wafers. There is also a design rule based inspection, in which the dye has to fulfill some line width and spacing requirements, and feature shapes should fit predefined shapes. Examples of these inspection methods, and relevant apparatus and circuitry for implementing these methods, are described in various U.S. patents, including, inter alia, U.S. Pat. Nos. 4,805,123; 4,926,489; 5,619,429; and 5,864,394. The disclosures of these patents are incorporated herein by reference.
Known inspection techniques typically use imaging the objects with a large magnification onto a charge-coupled device (CCD) camera. The imaging technique requires the object to be illuminated. The brightness of the illuminating source is a key factor in the ability to speed the inspection by reducing the integration time of the camera. As the patterns on wafers get smaller, it becomes necessary to use smaller wavelengths in order to be able to detect the patterns. This is due to the fact that the physical resolution limit depends linearly on the illumination wavelength and due to interference effects which require that the inspection be done at a wavelength similar to the one used in the lithographic process. As the wavelengths get smaller, conventional incoherent light sources like filament lamps or gas discharge lamps do not have enough brightness, and the light sources of choice become short wavelength lasers. The coherence of the laser, together with the roughness and aberrations of the surfaces as well as the patterned object along the light path, creates an artifact known as speckle, which is a noisy pattern over the image of the object.
Speckle causes problems in detection of the surfaces of objects being inspected and causes false alarms because of the non uniformity of the light pattern hitting the detector. Detection accuracy is degraded. Also, images taken of inspected objects are degraded. The problem is an acute one in this type of object inspection, because the power provided by coherent light is essential, among other reasons, as a result of losses stemming from the detection process.
When inspecting objects such as wafers or reticles, the speckle phenomenon can arise, if the spot size used for illumination is not much smaller than an element of a pattern on the wafer. However, in some circumstances, such as oblique illumination (in which the coherent light source is directed to the wafer at an angle), the spot size will be sufficiently large to cause speckle. Reducing the spot size will reduce system throughput and will require working at a wavelength that is smaller and different from the one used for imaging the object for example during the lithographic process. Consequently, as can be appreciated, there is a tradeoff between enduring speckle and optimizing detection sensitivity/throughput. Therefore, it would be desirable to solve the speckle problem, and thus enable the use of an increased spot size, and thus improve throughput.